In general, and there could be may exceptions depending on exactly how yours is plumbed, you just open the fill valve while watching the pressure gauge. When the pressure gets up to about 15psi as read on the gauge, you can turn off the feed and turn on the boiler. With the thermostat set for heat, the circulator should turn on. Double check the pressure, and add water to keep it up at 15psi or whatever your 'normal' pressure is. You're likely to need to go the the highest point(s) in the system and open the purge valves to let air out. You may have to do that more than once, as water from the tap used to fill it has some trapped air in it that will find its way to the highest points. This is a VERY general set of instructions. You don't want to turn things on until you have water in the boiler. The circulator doesn't last long if it's trying to pump air. If you don't have a working purge valve (something like a Spirovent or a 'tophat'), there'd be no way for the air to get out, so you'd not get much water in before the pressure showed up at 15psi. You can normally hear the water getting in and get an idea when it's getting full from the sound it makes.